The present invention relates to a residual current device.
A residual current device is used to ensure protection against a dangerous fault current in an electrical system. Such a fault current occurs when a live conductive part makes an electrical contact with ground. This occurs, for example, when somebody touches a live part of an electrical system. The fault current then flows via the person to ground, as a body current. The residual current devices which are used for protection against dangerous body currents have to isolate the electrical system from the mains voltage in the event of a fault current that is greater than 30 mA.
The design of a residual current device is described, for example, in the article xe2x80x9cWarum Fehlerstrom-Schutzschalter mit netzspannungsunabhxc3xa4ngiger Auslxc3x6sung?,xe2x80x9d etz, Volume 107 (1986), Issue 20, pages 938 to 945. Outline circuit diagrams and functional principles of a residual current device are described there, in particular in FIGS. 1 to 3. In this case, a distinction is drawn between two different basic types. The residual current device (RED) is a fault-current protective device in which the electrical power required for the switching process is obtained from the fault current itself, independently of the mains voltage. A differential-current or DI circuit breaker is, in contrast, a fault-current protective device in which the electrical auxiliary energy required for the switching process is taken from the mains voltage itself. Such a DI circuit breaker thus requires a mains connection and a power supply unit to operate it. The power supply unit converts the mains voltage into the supply voltage required to operate the DI circuit breaker components.
In such a DI circuit breaker, the voltage induced by the fault current in the secondary winding of the core-balanced transformer is normally amplified in an amplifier. If the fault voltage that is present at the output of the amplifier exceeds a predetermined reference voltage, then the DI circuit breaker trips. This reference voltage is generally provided by a voltage divider that is connected to a power supply unit, which supplies the electronic circuits located in the DI circuit breaker with the operating voltage required to operate it.
Any fault current or dissipation current flowing in an electrical system that is less than the tripping fault current leads to a fault voltage at the output of the amplifier, which is less than the reference voltage associated with the tripping fault current. If the mains voltage is switched off, then the operating voltage falls, and the reference voltage thus also falls, with a time constant that is dependent on the magnitude of the smoothing capacitors. If the reference voltage is less than the fault voltage at a time at which sufficient energy is still stored in the power supply unit to trip the tripping relay, the DI circuit breaker trips incorrectly.
An object of the present invention is a residual current device that avoids a fault operating state caused by the mains voltage being switched off.
The present residual current device according to the invention contains an amplifier circuit for producing a fault voltage that is derived from a fault current, a comparison circuit for comparing the fault voltage with a reference voltage, and a reference circuit, which is connected to a power supply unit, for producing the reference voltage as well as an operating voltage for the amplifier circuit, in which case means are provided which, if the output voltage from the power supply unit falls below a predetermined value, raise the reference voltage at least to the instantaneous operating voltage, at least if the fault voltage is less than the reference voltage.
In other words, if the fault voltage is less than the reference voltage at the time at which the output voltage from the power supply unit falls below the predetermined value, that is to say when the tripping condition for the tripping circuit of the residual current device is not satisfied at this time, the reference voltage is increased at least to the instantaneous operating voltage, which is in the process of falling.
Raising the reference voltage at least to the instantaneous operating voltage, preferably to the instantaneous operating voltage, ensures that the fault voltage derived from the fault current by the amplifier circuit is not less than the reference voltage as the output voltage of the power supply unit falls, since the output voltage of an amplifier circuit is, as a rule, less than the operating voltage used to operate it. This largely avoids incorrect tripping when the mains voltage is switched off.
In a further advantageous refinement of the present invention, the reference circuit contains a first voltage divider, which is connected to the operating voltage, for producing the reference voltage. That first voltage divider is connected to ground via a switching element which can be controlled as a function of the output voltage from the power supply unit. In this way, the reference voltage can be raised to the operating voltage by a simple switching process.
A transistor is preferably provided as the controllable switching element.
In a further advantageous refinement of the invention, the reference circuit contains a second voltage divider for producing a control voltage for the controllable switching element.
The second voltage divider is connected between the output voltage of the power supply unit and ground, and preferably includes a zener diode, which is connected in series, in the reverse direction, between the divider resistors of the second voltage divider, and to the base of a transistor, which is provided as the controllable switching element.
In a further embodiment of the present the invention, the output of the comparison circuit is connected to the reference circuit via a feedback circuit. This ensures that, in the event of a discontinuity in the supply voltage provided by the power supply unit, which may have been caused by actuating a tripping coil, the tripping condition required for the tripping coil to trip, namely that the fault voltage is greater than a reference voltage, is maintained.
In particular, the feedback circuit contains a diode and a resistor connected in series with it, and is connected to a control line which carries the control voltage.
In a further advantageous embodiment, the reference circuit contains a voltage stabilization circuit for producing a stabilized operating voltage from the output voltage. This creates reproducible and stable operating conditions for the residual current device.